Voltage-gated sodium (NaV) channels are responsible for the upstroke of the action potential in most excitable cells, including nerve cells [Hille, B. Ion channels of excitable membranes. (2001), 3rd ed, Sinauer Associates, Sunderland, Mass.]. NaV channels open in response to membrane depolarization and generate an inward current that underlies the upstroke of the action potential. In general, NaV channels open quickly (within msec) in response to depolarization and then just as rapidly close by a process called inactivation. Thus, these channels can exist in several different conformations or ‘states’ whose occupancy is governed by membrane voltage.
NaV channels are composed of a pore-forming alpha subunit responsible for ion conduction and gating [Catterall, W A, J. Physiol. 590(11): 2577-2599, (2012)]. These large single polypeptides (>250 kDa) are organized into four functional domains (DI-DIV), each with 6 transmembrane segments (S1-S6). Each domain can be further subdivided into the voltage-sensor domain (VSD) comprised of segments S1-S4 and the pore domain comprised of segments S5-S6. In addition to the alpha subunit, NaV channels have associated beta subunits which have a single transmembrane segment and a large extracellular immunoglobin-like region. Beta subunits modulate expression, gating and localization of the alpha subunit and interact with the extracellular matrix and intracellular cytoskeleton [Isom, L L, Neuroscientist, 7(1):42-54, (2001)].
Nine mammalian NaV alpha subunit genes exist. Based on the established nomenclature, they are referred to as NaV1.1-NaV1.9 [Goldin, A L et al., Neuron 28(2): 365-368, (2000)]. In addition to the primary sequences and homology, individual NaV1 family members are characterized by specific gating properties, localization and pharmacology [Catterall, W A, Goldin A L and SG Waxman, Pharmacol. Rev. 57(4):397-409, (2005)]. For example, NaV1.5 is expressed almost exclusively in the heart and is weakly sensitive to the neurotoxin tetrodotoxin (TTX). In contrast, NaV1.7 is mostly expressed in peripheral sensory neurons and is TTX-sensitive. A second sub-family of NaVs channels (NaV2/NaG) also exists [Wantanabe, E et al., J. Neurosci., 20(20):7743-7751, (2000)].
Several sites of drug action on NaV channels are known, based primarily on mutagenesis studies. For example, local anesthetic molecule binding has been mapped to specific residues on the S6 segment of DI, DIII and DIV [Ragsdale, D S et al. Science 265(5179):1724-1728, (1994); Ragsdale D S et al., Proc. Natl. Acad. Sci. USA 93(17):9270-9275; Yarov-Yarovoy, V et al., J. Biol. Chem. 276(1):20-27, (2001); Yarov-Yarovoy, V et al., J. Biol. Chem. 277(38):35393-35401, (2002)]. Six neurotoxin receptor sites (Sites 1-6) on NaV channels have been identified (reviewed in [Catterall, W A et al., Toxicon 49(2):124-141, (2007)]). Site 1 binds the pore-blockers tetrodotoxin and saxitoxin and is formed by residues of the pore loops of all four domains [Noda, M et al., FEBS Lett. 259(1):213-216, (1989); Terlau, H et al., FEBS Lett. 293(1-2):93-96, (1991)]. Site 2 binds lipid soluble toxins like veratridine and batrachotoxin and maps to S6 residues in D1 and DIV [Trainer, V L et al., J. Biol. Chem. 271(19):11261-11267, (1996); Kimura, T et al. FEBS Lett. 465:18-22, (2000)]. Alpha scorpion toxins bind to Site 3 which includes the S3-S4 loop of DIV [Rogers, J C et al., J. Biol. Chem. 271: 15950-15962, (1996)]. Site 4 binds beta scorpion toxins and includes the S3-S4 loop of DII [Cestele, S et al., J. Biol. Chem. 282:21332-21344, (1998)]. Site 5 is where the so-called red-tide toxins like brevetoxin bind and includes the S6 of D1 and S5 of DIV [Trainer, V L et al., Mol. Pharmacol. 40(6):988-994, (1991); Trainer, V L et al., J. Biol. Chem. 269(31):19904-19909, (1994)]. Delta-conotoxins bind to Site 6 which includes residues in S4 of DIV [Leipold, E, et al., FEBS Lett 579(18):3881-3884, (2005)].
Significant genetic data points to a role of NaV1.7 (SCN9A) in human pain perception. Most dramatically, rare mutations in SCN9A which result in loss-of-function of NaV1.7 protein cause congenital insensitivity to pain (CIP) in humans [Cox, J J et al., Nature 444(7121): 894-898, (2006); Goldberg, Y P et al., Clin. Genet. 71(4):311-319, (2007); Ahmad, S et al., Hum. Mol. Genet. 16(17): 2114-2121, (2007)]. These patients have normal intelligence but are unable to sense pain, even to stimuli which case significant injury. The only other significant deficit in these patients is anosmia, presumably due to a role of NaV1.7 in olfaction. Studies in genetically modified mice also point to a key role of NaV1.7 in pain perception. Deletion of Nav1.7 in both sensory and sympathetic neurons of mice abolishes mechanical, inflammatory and neuropathic pain responses [Minett, M S et al., Nat. Commun. 3:791, (2012)]. Recently, global gene disruption of SCN9A in mice has been reported to recapitulate the CIP phenotype [Gingras, J et al. PLoS One 9(9): e105895, (2014)]. Furthermore, inducible deletion of NaV1.7 in DRGs of adult mice reverses neuropathic pain [Minett, M S et al., Cell Rep. 6(2): 301-312, (2014)], suggesting that pharmacological inhibition of NaV1.7 channels in humans will be analgesic. In addition to the compelling evidence from these loss-of-function studies, spontaneous inherited pain syndromes in humans have been linked to gain-of-function of NaV1.7. Specifically, three syndromes in humans are linked to mutations in SCN9A: inherited erythromelalgia (IEM) [Yang, Y et al., J. Med. Genet. 41(3): 171-174, (2004)], paroxysmal extreme pain disorder (PEPD) [Fertleman, C R et al., Neuron 52(5):767-774, (2006)] and small fiber neuropathy (SFN) [Faber, C G et al. Ann. Neurol. 71(1): 26-39, (2012)]. In general, mutations in SCN9A linked to IEM result in enhanced channel activation where PEPD mutations result in impaired fast inactivation (reviewed in [Dib-Hajj, S D et al., Nat. Rev. Neurosci. 14(1): 49-62, (2013)]). Mutations linked to SFN alter fast inactivation and/or slow inactivation [Faber, C G et al. Ann. Neurol. 71(1): 26-39, (2012)].
Given the importance of NaV1.7 in pain perception, considerable effort has been expended to identify selective inhibitors of the channel. Peptides identified from venom are common sources of potent ion channel modifiers. For NaV1.7, the peptide ProTx-II from tarantula was first identified as an inhibitor of NaV1.8 [Middleton, R E et al. Biochemistry 41(50): 14734-14747, (2002)] and later found to be approximately 100-fold selective for NaV1.7 over other NaV channels [Schmalhofer, W A et al. Mol. Pharmacol. 74(5): 1476-1484, (2008)]. ProTx-II binding determinants are primarily in the VSD of DII and DIV whereas the related peptide, Huwentoxin-IV, is thought to interact primarily with the DII VSD [Xiao, Y et al., Mol. Pharmacol. 78(6): 1124-1134, (2010)]. Extensive structure-activity studies of ProTx-II have yielded peptides with potencies in the picomolar range [Park, JH et al. J. Med. Chem. 57(15): 6623-6631, (2014)]. Structure-based engineering of another tarantula peptide, GpTx-1, has yielded peptides with optimized potency and selectivity [Murry, JK et al., J. Med. Chem. 58(5): 2299-2314, (2015)].
Efforts to identify small molecular weight inhibitors of NaV1.7 have been extensive. Numerous NaV1.7 blockers have been reported in the literature (reviewed in [de Lera Ruiz, M and RL Kraus, J. Med. Chem. 58(18) 7093-7118, (2015)]) although most do not have sufficient selectivity over other NaV subtypes. A significant advance came with the discovery of a class of arylsulfonamides with subtype selectivity [McCormack, K et al., Proc. Natl. Acad. Sci. USA, 110(29): E2724-E2732, (2013)]. Some members of the series include molecules that are highly selectivity for NaV1.7. Three residues in the VSD of DIV were identified as conferring potent inhibition by one such molecule, PF-04856264. The recent co-crystal structure of a chimeric channel consisting of a portion of the NaV1.7 DIV VSD grafted onto the bacterial NaV channel NavAb with a related arylsulfonamide bound defines some of the primary interactions between this class of molecules and the NaV1.7 DIV VSD [Ahuja S, et al., Science 350(6267): aac5464, (2015)]. These studies point to the possibility of discovering highly potent and selective inhibitors of NaV1.7 with properties suitable for use as oral analgesics.